1. Technical Field
The present invention relates to a technology of controlling a unit inverter system where a dispersed power source whose power fluctuates, such as in solar battery or wind power generation, includes multiple unit inverters, and a quantity of operating unit inverters is changed in accordance with power generated. The present invention also relates to a dead time regulator that adjusts dead time of a unit inverter to reduce an output current of the unit inverter before stopping the unit inverter to reduce the quantity of operating unit inverters.
2. Related Art
The configuration of a unit inverter system using heretofore known technology, where multiple inverter units are connected in parallel, and the capacity of the system is increased, is shown in FIG. 6. The system includes a direct current power source 1 such as a solar battery as an input, an inverter INV1, whose output is connected to an alternating current filter formed of an alternating current reactor L1 and a capacitor C10, a balance reactor BL1 and a contactor CN1. The system also includes an inverter INV2, whose output is connected to an alternating current filter formed of an alternating current reactor L2 and a capacitor C20, a balance reactor BL2, and a contactor CN2. The inverters INV1 and INV2 are connected in parallel, and connected to an alternating current power source ACP of the system. The inverters INV1 and INV2 respectively include current detectors CT1 and CT2, which detect an output current, and dead time regulators DTR1 and DTR2. The output current of each inverter is controlled by a command from a common controller CONT.
When connecting this kind of unit inverter to a system, it is connected to the system after reducing a high frequency current output by the inverters using the inverters, the alternating current reactors, and the capacitors. Furthermore, when providing a contactor in each unit, the balance reactors BL1 and BL2 are used to balance the current of the contactors, in order to reduce the cost of the contactors.
This kind of unit inverter calculates an average current of each unit using the circuit configuration shown in FIG. 8 and principle shown in FIG. 9 described in JP-A-2006-296110, and carries out a control using a voltage at a connection point with the current. Referring to FIG. 6, a PWM pulse signal output by the controller CONT is supplied to the inverters as a gate signal via the dead time regulators DTR1 and DTR2 in each unit. The average current can be obtained using the kind of calculator circuit shown in FIG. 9 by converting the output of each current detector into a voltage, as described in JP-A-2006-296110.
The configuration of the dead time regulators DTR1 and DTR2 is shown in FIG. 7. The dead time regulators adjust dead time in such a way that an own unit current amplitude value of each unit calculated by an amplitude calculator AC1, and an in-unit average current amplitude value calculated by an amplitude calculator AC2, match. The own unit current output by each unit, and the average current amplitude value, are calculated by the amplitude calculators AC1 and AC2. A deviation of the two current values is obtained by an adder AD1, and input into an regulator ACR. The output of the regulator ACR is input into an upper and lower limit limiter LMT which has 0 as a lower limit value, and a fixed value as an upper limit value. The output of the limiter LMT is then added by an adder AD2 to a dead time fixed value for preventing a short circuit of upper and lower arms of the inverters. The added value is used as a delay time of on-delay circuits DT1 and DT2 into which a gate signal and an inverted gate signal are input.
When using the unit inverter in a dispersed power source connected to a power generation source whose power constantly fluctuates, such as in solar light or wind power generation, it often happens that the converted power thereof is smaller than the device capacity. In this case, it is known that it is possible, without operating all of the units, to improve efficiency from the power generation source to the alternating current output by changing the quantity of units operated in accordance with the amount of power to be converted (refer to JP-A-61-135365, and the like). In this case, a method is adopted whereby the contactor is maintained in an on condition in order to prevent a reduction of lifespan, and only the gate signal of the inverter is put into a locked condition.
Herein, in the case of stopping the gate signal of one portion of the operating units, there is a problem in that the balance reactor and filter capacitor connected to the relevant unit form a resonant circuit, and the resonant current thereof has an adverse effect on the system. Even in a hypothetical case in which no contactor is used, and no balance reactor is provided, when the wiring from the connection point of each unit to the filter capacitor is long, the impedance thereof acts as inductance, meaning that a resonant current is generated.